1. Field of the Invention
The present invention relates to a phase-locked loop device (referred to as a PLL hereinafter) for use in a video intermediate frequency amplifying system (referred to as a VIF system hereinafter) and the like, and to a voltage-controlled oscillator (referred to as a VCO hereinafter) for use in the PLL.
2. Description of the Background Art
Recently, there have been increasing demands toward reduction in the number of parts to be adjusted in chassis of TVs and VTRs for the purpose of rationalization. Requirements have accordingly been increased for non-adjustment of exteriorly attached parts for VIF-ICs used in the VIF system. In the past, a PLL has been used in the VIF system, and improvements in video detection and demodulation accuracy have been intended using a synchronization maintaining characteristic of the PLL. For non-adjustment of the exteriorly attached parts of the PLL, it has however been necessary to meet two inconsistent conditions: reduction in free-running frequency variations upon no signal input; and a wide synchronization maintaining range (referred to as a lock range hereinafter).
FIG. 17 illustrates a conventional PLL of an adjustment type used for the VIF system.
In FIG. 17, the reference numeral 1 designates an input terminal receiving an amplified signal which is applied to the VIF system; 2 designates a phase comparator for comparing the phase of the amplified signal with the phase of an oscillator to be described later of the PLL to output an error signal through an output terminal 3; 4 designates a low-pass filter (referred to as a LPF hereinafter) for removing a high-frequency component superimposed on the error signal and passing a frequency area required for the PLL to be stably locked; 6 designates an oscillator which receives the output from the LPF 4 through a voltage-controlled terminal 5 and varies the frequency in accordance with a control voltage applied to the voltage-controlled terminal 5; and 8 and 9 designate a capacitor and a coil, respectively, for determining the free-running frequency of the oscillator 6 and connected between terminals 7a and 7b.
The reference character 10a designates a VCO which outputs the varied frequency as an output signal to the phase comparator 2. Then, a loop based on the phase difference between the amplified input signal and the output signal from the VCO 10a is locked.
The reference character 100a designates portions provided in the form of an integrated circuit.
The background art coil 9 is equipped with an adjusting terminal. By mechanically changing the adjusting terminal, the free-running oscillation frequency of the oscillator 6 may be arbitrarily changed, providing a wide lock range.
The adjustment type PLL is disadvantageous in that it is necessary to change the adjusting terminal mechanically.
FIG. 18 illustrates another conventional PLL which solves the problem of the foregoing PLL.
In FIG. 18, the reference numerals 1 to 5 designate parts identical with those of the PILL of FIG. 17. The reference numeral 11 designates a phase changer for changing the phase by using the control voltage applied to the voltage-controlled terminal 5; and 12 designates an amplifier for changing the phase in accordance the amount of oscillation frequency changes. The amplifier 12 is connected through an input terminal 13 to a quartz oscillator 14 serving as an oscillation source for determining the free-running oscillation frequency.
The reference character 10b designates a VCO for feeding the changed frequency back to the phase changer 11 therein and outputting the changed frequency to the phase comparator 2 as an output therefrom. Then, a loop based on the phase difference between the amplified input signal and the output signal from the VCO 10b is locked.
The reference character 100b designates portions provided in the form of an integrated circuit.
Characteristics of the conventional phase changer 11 are shown in FIG. 19.
As shown, the phase changer 11 has the characteristic of providing an output signal having a constant amplitude and a phase varying from +90.degree. to -90.degree. depending upon a voltage applied to the voltage-controlled terminal 5.
Characteristics of the amplifier 12 are shown in FIG. 20.
As shown, the amplifier 12 has the characteristic of providing an output signal having a constant amplitude and a phase varying depending upon the oscillation frequency.
Operation of the conventional VCO 10b of FIG. 19 will be discussed below with reference to FIGS. 19 and 20. The circuit of FIG. 18 oscillates at a frequency which provides the 360.degree. (0.degree.) phase of a loop composed of the amplifier 12 and the phase changer 11. Changes in phase amount of the phase changer 11 by changing the control voltage applied to the voltage-controlled terminal 5 can change the oscillation frequency of the VCO 10b of FIG. 18.
For example, the phase is .theta.2 (=0.degree.) when a voltage Vo is applied to the voltage-controlled terminal of the phase changer 11 having the control characteristic shown in FIG. 19, and the amplifier 12 oscillates at an oscillation frequency fo when the amplifier 12 has a phase .theta.1 (=0.degree.) shown in FIG. 20. At this time, .theta.1+.theta.2=360.degree. (0.degree.) is satisfied. Then, if the voltage at the voltage-controlled terminal of the phase changer 11 is changed to V1 as shown in FIG. 19 to lead the phase .theta.2 of the phase changer 11 by .DELTA..theta., the phase of the amplifier 12 is changed to .theta.1-.DELTA..theta. which satisfies .theta.1+.theta.2=360.degree. (0.degree.) and the amplifier 12 oscillates at a frequency f1 which provides the phase .theta.1-.DELTA..theta. as shown in FIG. 20.
The conventional VCOs for the PLL are constructed as above described. In the PLL of FIG. 17, the oscillation frequency upon no signal input is lacking in accuracy and is required to be necessarily adjusted when the capacitor 8 and the coil 9 are made of discrete parts.
In the PLL of FIG. 18, if a very small input signal is applied to the terminal 1, DC offset of the phase comparator 2 is applied through the LPF 4 to the control terminal 5 of the phase changer 11 as an offset voltage, which causes the free-running oscillation frequency (an oscillation frequency upon no signal input) to be shifted.
In the PLL of FIG. 18, since the variable phase range of the phase changer 11 is .+-.90.degree. as shown in FIG. 19, the amplifier 12 can oscillate in the range of fmin to fmax corresponding to the phases .+-.90.degree. of FIG. 20, resulting in the variable phase range of the phase changer being equal to the phase range of the amplifier 12.
This is apparent from the loci of the output vectors of the phase changer 11 and the amplifier 12 shown in FIG. 21.